Nitrogen isotopic studies in the suboxic Arabian Sea

Abstract

Measurements of ¹⁵N/¹⁴N in dissolved molecular nitrogen (N₂), nitrate (NO₃^- ) and nitrous oxide (N₂O) and ¹⁸O/¹⁶O in N₂O [expressed as δ¹⁵N and δ¹⁸O, relative to atmospheric N₂ and oxygen (O₂), respectively] have been made in water column at several locations in the Arabian Sea, a region with one of the thickest and most intense O₂ minima observed in the open ocean. Microbially-mediated reduction of NO₃^- to N₂ (denitrification) in the oxygen minimum zone (OMZ) appears to greatly affect the natural isotopic abundances. The δ¹⁵N of NO₃^- increases from 6‰ in deep waters (2500 m) to 15‰ within the core of the denitrifying layer (250-350 m); the δ¹⁵N of N₂ concurrently decreases from 0.6‰ to 0.20‰ Values of the isotopic fractionation factor (e) during denitrification estimated using simple advection-reaction and diffusion-reaction models are 22‰ and 25‰, respectively. A strong decrease in δ¹⁵N of NO₃^- is observed from ~ 200m (> 11‰) to 80m (~ 6‰); this is attributed to the input of isotopically light nitrogen through nitrogen fixation. Isotopic analysis of N₂O reveals extremely large enrichments of both ¹⁵N and ¹⁸O within the OMZ, presumably due to the preferential reduction of lighter N₂O to N₂. However, isotopically light N₂O is observed to accumulate in high concentrations above the OMZ indicating that the N₂O emitted to the atmosphere from this region cannot be very heavy. The isotope data from the intense upwelling zone off the southwest coast of India, where some of the highest concentrations of N₂O ever found at the sea surface are observed, show moderate depletion of ¹⁵N, but slight enrichment of ¹⁸O relative to air. These results suggest that the ocean-atmosphere exchange cannot counter inputs of heavier isotopes (particularly ¹⁸O) associated with the stratospheric back flux, as proposed by previous workers. This calls for additional sources and/or sinks of N₂O in the atmosphere. Also, the N₂O isotope data cannot be explained by production through either nitrification or denitrification, suggesting a possible coupling between the two processes as an important mechanism of N₂O production.